This invention pertains generally to automatic control systems for steam turbines and in particular to apparatus for insuring the utmost reliability in signals which are generated as representative of turbine operating parameters and which serve as inputs to a turbine control system.
It is an elementary proposition that control systems for large steam turbines are based upon some continuously measured operating parameter of the turbine. For example, in the ubiquitous speed control system it is obvious that some actual speed responsive means must be provided to continuously inform the attendant controller of any need for corrective action. It becomes apparent then, regardless of which particular variable is under control, that the reliability of the signals representing the measured quantity is of critical importance if the turbine is to be properly controlled.
One conventional method of improving control signal reliability is to use multiple redundant sensing devices so that, by judicious choice on the part of operating personnel, at least one sensor is always available for control.
However, in many cases, particularly in steam pressure control loops, it often becomes necessary, because of sensor failure, for the operator to rely upon manual control. This, of course, requires continuous close attention on the part of the operator during the period of manual control with the result that pressure control is not as effective as is frequently desired.
With the current interest in operating steam turbines in the steam bypass mode, the importance of achieving good automatic control of the main and reheated steam pressures has assumed increased importance. Generally, in this mode of operation control of the bypass valving is intimately related to measurements of these steam parameters. For example, in U.S. Pat. No. 4,253,308 to Eggenberger et al. a bypass control system is disclosed in which fundamental control decisions are based on measurements of the main and reheated steam pressures. Further, in U.S. Pat. No. 4,357,803 filed in the name of the present inventor and of common assignee with the present invention, a comprehensive bypass control system is disclosed which relies upon the fundamental pressure measurement to the extent that manual/automatic control (M/A) means are entirely eliminated from the bypass control loops. If such reliance is not to be misplaced, extraordinary measures are required in these pressure control systems to guard against loss of the input pressure signals. Perhaps more damaging, there is the possibility of basing the control decisions on an incorrect measurement of the steam pressure.
Accordingly, it is an object of the present invention to provide apparatus which assures reliability of the signal representing a controlled variable in the operation of a large steam turbine.
Another, more particular object of the invention is to provide an automatic signal selector for assuring selection of a valid signal representation of steam pressure for utilization by a turbine control system wherein steam pressure is a controlled parameter.
Other objects and advantages of the invention will be apparent from the following description of a preferred form of the invention and its operation.